Comparative Physiology. 395 



ring. This was probably, however, owing to the less expansion 

 by growth above than below causing less wood, and of more 

 specific gravity. It is not at all probable that more sap would 

 be conveyed to a ringed than an unringed branch ; and the 

 truth of less sap producing fruitfulness is undoubted, from dry 

 poor soils producing fruitfulness much sooner than rich moist 

 soils, and the lopping off of large roots having the same effect. 

 The healthy strong-growing plant in which the nutritive func- 

 tion is in full vigour, not plethoric, produces, however, always 

 the best fruit, though not the greatest quantity of it : the seeds 

 also from such trees will furnish the most healthy strong seed- 

 lings ; and perhaps the furnishing of a limited supply of food, and 

 depressing the vigour of the nutritive system, are more pro- 

 ductive of precocity than real fruitfulness. The two functions 

 are undoubtedly in so far antagonist, that the one can only 

 subsist at the expense of the other ; but this would rather seem 

 to imply a necessity of vigour in the nutritive to supply the 

 wants of the other. Plethora depresses the vigour of the nu- 

 tritive function, and, in animals at least, may weaken by exces- 

 sive stimuli as well as disease. Plants possess a corrective to 

 plethora in the extension of the system, which is not possessed 

 by animals. There are no determinate limits to their extension : 

 and the addition of new shoots and branches is a means of 

 getting rid of excess in the quantity of food, which must tend to 

 make the plethoric state less frequent, aAd make amends for the 

 want of a sensitive appetite to determine the quantity absorbed. 

 Trees that are found plethoric and apt to canker in cold wet sea- 

 sons, become again sound when warm congenial seasons ensue ; 

 much is therefore owing to want of heat and light, to which there 

 is also a corrective in the diminished quantity of endosmose ab- 

 sorption occasioned by their absence. When so many causes are at 

 work, it requires great skill and consideration in the operative 

 to apply the necessary correctives, at the proper times, and in 

 the proper manner ; and, after all, his efforts may be baffled by 

 unforeseen and unexpected alterations of the weather. 



I have before noticed that when we consider the soil 

 as the stomach of the jolant, in which the food is digested by a 

 process of solution similar to that in the stomach of animals, 

 the manner of performing the next function, absorption, seems 

 to differ little in character in plants from that of animals. The 

 function of circulation appears undoubtedly to be more under 

 the power of vital contractile force in animals, and to be more 

 under the influence of external agents in plants. Though the 

 power of endosmose, said to be the principal cause of circulation 

 in plants, does not seem much dependent on vitality, and rather 

 depends on heat and light for a continuation of its action, it is 

 the opinion of the best of vegetable physiologists, that this power 



